Cancer diagnosis and treatment is one of the most difficult problems facing clinical nuclear medicine today. This is due to the malignancy of some types of tumors, that is, their ability to spread to other parts of the body. By the time a primary tumor can be located it may already have metastasized establishing new colonies of tumor cells throughout the system. It is important to be able to locate these new colonies for it is these metastasis which usually prove to be fatal, not the primary tumor. Certain antibodies can be found with their corresponding antigens at the tumor site. If these antibodies could be labeled with a gamma emmitting radionuclide it would allow the use of existing imagining instruments and techniques to trace the location of the new colonies.
Radiolabeling of antibodies with iodine-131 and iodine-123 has been attempted in the past using what is referred to as the Chloramine-T procedure or a modification of this method. It is believed that the phenyl group of the tyrosine residues in the protein contain the radiolabel. Drawbacks of this procedure include substrate exposure to chemicals during radioiodination and the possibility of non-specific radiolabeling and denaturation of the protein resulting in low yields of the radiolabeled antibody. Another major drawback is in vivo deiodination of the antibody and entrapment of free radioiodine in the stomach mucosa and decreased tissue half-life of the radiolabeled antibody. Therefore, there is a need to develop techniques that provide specific radiolabeling of proteins with a minimum of in vivo chemical degradation of the radiopharmaceutical.